U.S. patent application number 10/925244 was filed with the patent office on 2006-03-02 for wet chemical method to form silver-rich silver-selenide.
Invention is credited to Kristy A. Campbell, Rita J. Klein.
Application Number | 20060045974 10/925244 |
Document ID | / |
Family ID | 35943553 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060045974 |
Kind Code |
A1 |
Campbell; Kristy A. ; et
al. |
March 2, 2006 |
Wet chemical method to form silver-rich silver-selenide
Abstract
A method of forming a silver-rich silver-selenide layer is
provided. The method includes plating a silver layer on a
silver-selenide layer using an electroless process and diffusing
silver into the silver-selenide layer. Also, a method of forming a
memory element is provided. The memory element is formed by forming
a first electrode and forming a first layer of resistance variable
material over the first electrode. A silver-selenide layer is
formed over the first layer of resistance variable material and a
silver layer is plated on the silver-selenide layer by an
electroless process.
Inventors: |
Campbell; Kristy A.; (Boise,
ID) ; Klein; Rita J.; (Boise, ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L STREET, NW
WASHINGTON
DC
20037
US
|
Family ID: |
35943553 |
Appl. No.: |
10/925244 |
Filed: |
August 25, 2004 |
Current U.S.
Class: |
427/304 ;
257/E45.002; 427/76 |
Current CPC
Class: |
C23C 14/0623 20130101;
C23C 18/165 20130101; C23C 18/44 20130101; H01L 45/1625 20130101;
H01L 45/1233 20130101; H01L 45/085 20130101; H01L 45/1641 20130101;
H01L 45/143 20130101; C23C 18/1692 20130101; C23C 16/305 20130101;
H01L 45/1616 20130101 |
Class at
Publication: |
427/304 ;
427/076 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Claims
1. A method of forming a resistance variable memory element, the
method comprising the steps of: forming a first electrode; forming
a layer of resistance variable material over the first electrode;
forming a silver-selenide layer over the first layer of resistance
variable material; and plating a first silver layer on the
silver-selenide layer by an electroless process.
2. The method of claim 1, wherein the plating act comprises plating
the first silver layer having a thickness within the range of
approximately 50 .ANG.to approximately 250 .ANG..
3. The method of claim 2, wherein the plating act comprises plating
the first silver layer having a thickness of approximately 200
.ANG..
4. The method of claim 1, further comprising the act of diffusing
silver from the first silver layer into the silver-selenide layer
to form a silver-rich silver selenide layer.
5. The method of claim 1, wherein the resistance variable material
is germanium-selenide glass having a Ge.sub.xSe.sub.100-x
stoichiometry.
6. The method of claim 1, further comprising the act of forming a
second electrode over the first silver layer.
7. The method of claim 5, further comprising forming a conductive
adhesion layer between the first silver layer and the second
electrode.
8. The method of claim 6, wherein the conductive adhesion layer and
the resistance variable material layer are a same material.
9. The method of claim 1, further comprising the act of forming a
second silver layer between the layer of resistance variable
material and the first electrode.
10. The method of claim 1, further comprising the act of activating
the silver-selenide layer prior to the step of plating.
11. The method of claim 10, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution.
12. The method of claim 11, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water, a water soluble silver comprising compound, a chelating
agent, and a reducing agent.
13. The method of claim 11, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water, ammonium hydroxide; ammonium sulfate; silver nitrate; and
tartrate.
14. The method of claim 13, wherein the plating solution further
comprises ammonium hypophosphite.
15. The method of claim 13, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
100 ml water; 5 ml ammonium hydroxide; 3 g ammonium sulfate; 1.25 g
silver nitrate; and 2 g tartrate.
16. The method of claim 11, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water; silver nitrate; ammonium hydroxide; acetic acid; and
EDTA.
17. The method of claim 16, wherein the plating solution further
comprises hydrazine.
18. The method of claim 11, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water; silver nitrate; succinimide; imidazole; and ammonium
hydroxide.
19. The method of claim 18, wherein the plating solution further
comprises hydrazine.
20. The method of claim 10, further comprising the act of
pretreating the silver-selenide layer prior to the act of
activating, the pretreating act comprising exposing the
silver-selenide layer to a mixture of ammonium fluoride and
phosphoric acid.
21. The method of claim 10, wherein the activating act comprises
exposing the silver-selenide to a nickel and gold colloidal
mixture.
22. The method of claim 21, wherein the exposing act is conducted
for approximately 5 minutes.
23. The method of claim 21, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 55.degree. C. for approximately 12
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
24. The method of claim 10, wherein the activating act comprises
exposing the silver-selenide to colloidal palladium particles.
25. The method of claim 24, wherein the exposing act is conducted
for approximately 5 minutes.
26. The method of claim 24, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
27. The method of claim 10, wherein the activating act comprises
exposing the silver-selenide to a solution of water, palladium
chloride and hydrogen fluoride
28. The method of claim 27, wherein the activating act comprises
exposing the silver-selenide to a solution of 500 ml water, 0.5 g
palladium chloride, and 1 ml hydrogen fluoride.
29. The method of claim 28, wherein the exposing act is conducted
for approximately 90 seconds.
30. The method of claim 28, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
31. The method of claim 30, wherein the plating solution further
comprises ammonium hypophosphite.
32. The method of claim 27, wherein the activating act further
comprises the act of exposing the silver-selenide layer to
dimethylethylenediamine.
33. The method of claim 32, wherein the act of exposing the
silver-selenide layer to dimethylethylenediamine is conducted prior
to the act of exposing the silver-selenide layer to the
solution.
34. The method of claim 33, wherein the act of exposing the
silver-selenide layer to dimethylethylenediamine is conducted for
approximately 2 minutes and the act of exposing the silver selenide
layer to the solution is conducted for approximately 1 minute.
35. The method of claim 34, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
36. A method of forming a memory element, the method comprising the
steps of: forming a first electrode; forming germanium-selenide
glass layer having a Ge.sub.xSe.sub.100-x stoichiometry over the
first electrode; forming a first silver-selenide layer over the
first layer of germanium-selenide; activating the silver-selenide
layer; plating a silver layer on the silver-selenide layer by an
electroless process subsequent to the activating act; forming a
second layer of germanium-selenide glass having a
Ge.sub.xSe.sub.100-x stoichiometry over the first silver layer;
forming a second electrode over the second germanium-selenide glass
layer; and forming an insulating layer over the second
electrode.
37. A method of forming a silver-rich silver selenide layer, the
method comprising the steps of: forming a silver-selenide layer;
plating a silver layer on the silver-selenide layer by an
electroless process; and diffusing silver from the silver layer
into the silver-selenide layer.
38. The method of claim 37, wherein the plating act comprises
plating the silver layer having a thickness within the range of
approximately 50 .ANG. to approximately 250 .ANG..
39. The method of claim 38, wherein the plating act comprises
plating the silver layer having a thickness of approximately 200
.ANG..
40. The method of claim 37, further comprising the act of diffusing
silver from the silver layer into the silver-selenide layer to form
a silver-rich silver selenide layer.
41. The method of claim 37, further comprising the act of
activating the silver-selenide layer prior to the step of
plating.
42. The method of claim 41, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution.
43. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water, a water soluble silver comprising compound, a chelating
agent, and a reducing agent.
44. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water, ammonium hydroxide; ammonium sulfate; silver nitrate; and
tartrate.
45. The method of claim 44, wherein the plating solution further
comprises ammonium hypophosphite.
46. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
100 ml water; 5 ml ammonium hydroxide; 3 g ammonium sulfate; 1.25 g
silver nitrate; and 2 g tartrate.
47. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water; silver nitrate; ammonium hydroxide; acetic acid; and
EDTA.
48. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
150 ml water; 0.81 g silver nitrate; 5.5 g ammonium hydroxide; 4.3
ml acetic acid; and approximately 0.2 to approximately 2 g
EDTA.
49. The method of claim 48, wherein the plating solution further
comprises hydrazine.
50. The method of claim 49, wherein the plating solution comprises
approximately 70 .mu.l of hydrazine.
51. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
water; silver nitrate; succinimide; imidazole; and ammonium
hydroxide.
52. The method of claim 42, wherein the exposing act comprises
exposing the silver-selenide layer to a plating solution comprising
150 ml water; 0.8 g silver nitrate; 2.4 g succinimide; 1.8 g
imidazole; and 1 ml ammonium hydroxide.
53. The method of claim 52, wherein the plating solution further
comprises hydrazine.
54. The method of claim 41, further comprising the act of
pretreating the silver-selenide layer prior to the activating act,
the pretreating act comprising exposing the silver-selenide layer
to a mixture of ammonium fluoride and phosphoric acid.
55. The method of claim 42, wherein the activating act comprises
exposing the silver-selenide to a nickel and gold colloidal
mixture.
56. The method of claim 55, wherein the exposing act is conducted
for approximately 5 minutes.
57. The method of claim 55, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 55.degree. C. for approximately 12
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
58. The method of claim 41, wherein the activating act comprises
exposing the silver-selenide to colloidal palladium particles.
59. The method of claim 58, wherein the exposing act is conducted
for approximately 5 minutes.
60. The method of claim 58, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
61. The method of claim 41, wherein the activating act comprises
exposing the silver-selenide to a solution of water, palladium
chloride and hydrogen fluoride
62. The method of claim 50, wherein the activating act comprises
exposing the silver-selenide to a solution of 500 ml water, 0.5 g
palladium chloride, and 1 ml hydrogen fluoride.
63. The method of claim 62, wherein the exposing act is conducted
for approximately 90 seconds.
64. The method of claim 62, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
65. The method of claim 64, wherein the plating solution further
comprises ammonium hypophosphite.
66. The method of claim 62, wherein the activating act further
comprises the act of exposing the silver-selenide layer to
dimethylethylenediamine.
67. The method of claim 66, wherein the act of exposing the
silver-selenide layer to dimethylethylenediamine is conducted prior
to the act of exposing the silver-selenide layer to the
solution.
68. The method of claim 67, wherein the act of exposing the
silver-selenide layer to dimethylethylenediamine is conducted for
approximately 2 minutes and the act of exposing the silver selenide
layer to the solution is conducted for approximately 1 minute.
69. The method of claim 68, wherein the plating act comprises
exposing the silver-selenide layer to a plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes, the plating solution comprising 100 ml water; 5 ml
ammonium hydroxide; 3 g ammonium sulfate; 1.25 g silver nitrate;
and 2 g tartrate.
70. A method of forming a silver-rich silver selenide layer, the
method comprising the steps of: forming a silver-selenide layer;
activating the silver-selenide layer; plating a silver layer on the
silver-selenide layer using an electroless process by exposing the
silver-selenide layer to a plating solution comprising water, a
water soluble silver comprising compound, a chelating agent, and a
reducing agent; and diffusing silver from the silver layer into the
silver-selenide layer.
71. A method of forming a silver-rich silver selenide layer, the
method comprising the steps of: forming a silver-selenide layer;
activating the silver-selenide layer; plating a silver layer on the
silver-selenide layer using an electroless process by exposing the
silver-selenide layer to a plating solution comprising water,
ammonium hydroxide; ammonium sulfate; silver nitrate; and tartrate;
and diffusing silver from the silver layer into the silver-selenide
layer.
72. The method of claim 71, wherein the activating act comprises
exposing the silver-selenide to a nickel and gold colloidal mixture
for approximately 5 minutes.
73. The method of claim 72, wherein the plating act comprises
exposing the silver-selenide layer to the plating solution at a
temperature of approximately 55.degree. C. for approximately 12
minutes
74. The method of claim 71, wherein the activating act comprises
exposing the silver-selenide to colloidal palladium particles for
approximately 5 minutes.
75. The method of claim 74, wherein the plating act comprises
exposing the silver-selenide layer to the plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes
76. The method of claim 71, wherein the activating act comprises
exposing the silver-selenide to a solution of 500 ml water, 0.5 g
palladium chloride, and 1 ml hydrogen fluoride.
77. The method of claim 76, wherein the exposing act is conducted
for approximately 90 seconds.
78. The method of claim 77, wherein the plating act comprises
exposing the silver-selenide layer to the plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes.
79. The method of claim 78, wherein the plating solution further
comprises ammonium hypophosphite.
80. The method of claim 76, wherein the activating act further
comprises the act of exposing the silver-selenide layer to
dimethylethylenediamine prior to the act of exposing the
silver-selenide layer to the solution.
81. The method of claim 80, wherein the act of exposing the
silver-selenide layer to dimethylethylenediamine is conducted for
approximately 2 minutes and the act of exposing the silver selenide
layer to the solution is conducted for approximately 1 minute.
82. The method of claim 81, wherein the plating act comprises
exposing the silver-selenide layer to the plating solution at a
temperature of approximately 60.degree. C. for approximately 10
minutes.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the field of random access memory
(RAM) devices formed using a resistance variable material, and in
particular to an improved method of manufacturing a resistance
variable memory element.
BACKGROUND OF THE INVENTION
[0002] A well known semiconductor memory component is a random
access memory (RAM). RAM permits repeated read and write operations
on memory elements. Typically, RAM devices are volatile, in that
stored data is lost once the power source is disconnected or
removed. Non-limiting examples of RAM devices include dynamic
random access memory (DRAM), synchronous dynamic random access
memory (SDRAM), and static random access memory (SRAM). DRAMS and
SDRAMS typically store data in capacitors that require periodic
refreshing to maintain the stored data. Although volatile, SRAMS do
not require refreshing.
[0003] Recently, resistance variable memory elements, which include
programmable conductor random access memory (PCRAM) elements, have
been investigated for suitability as semi-volatile and non-volatile
random access memory elements. Generally, a programmable conductor
memory element includes an insulating dielectric material formed of
a chalcogenide glass disposed between two electrodes. A conductive
material, such as silver, is incorporated into the dielectric
material. The resistance of the dielectric material can be changed
between high resistance and low resistance states depending upon
movement of the conductive material within or into and out of the
dielectric material in accordance with applied voltage.
[0004] One preferred resistance variable material comprises a
chalcogenide glass. A specific example is germanium-selenide
(Ge.sub.xSe.sub.100-x) containing a silver (Ag) component. One
method of providing silver to the germanium-selenide composition is
to initially form a germanium-selenide glass and then deposit a
thin silver layer upon the glass, for example by sputtering,
physical vapor deposition, or other known techniques in the art.
The silver layer is irradiated, preferably with electromagnetic
energy at a wavelength less than 600 nanometers, so that the energy
passes through the silver and to the silver/glass interface, to
break a chalcogenide bond of the chalcogenide material such that
the glass is doped or photodoped with silver. Another method for
providing silver to the glass is to provide a silver-selenide layer
on a germanium-selenide glass. A top electrode comprising silver is
then formed over the silver-germanium-selenide glass or, in the
case where a silver-selenide layer is provided over a
germanium-selenide glass, the top electrode is formed over the
silver-selenide layer.
[0005] It has been found that over time devices fabricated by the
above described methods may fail if excess silver from a top silver
containing electrode continues to diffuse into the silver
germanium-selenide glass or into the silver-selenide layer and
eventually into the germanium-selenide glass layer (the primary
switching area) below the silver-selenide layer. Furthermore,
during semiconductor processing and/or packaging of a fabricated
structure that incorporates the memory element, the element
undergoes thermal cycling or heat processing. Heat processing can
result in undesirable amounts of silver migrating into the memory
element. Too much silver incorporated into the memory element may
result in faster degradation, i.e., a short life, and eventually
device failure.
[0006] Typically, a chalcogenide-based programmable conductor
memory element is formed by depositing a silver layer onto a
silver-selenide layer to achieve a silver-rich silver-selenide
layer. For optimum cell operation, it is desirable to control the
amount of excess silver incorporated into the silver-selenide
layer. However, it is difficult to control the amount of silver
diffused into the silver-selenide layer when the silver layer is
deposited using sputter deposition or evaporation techniques since
these methods result in an unknown amount of silver being
incorporated into the silver-selenide layer.
[0007] Thus, there is a desire and need for a method of forming
silver-rich silver-selenide films for controlling the excess silver
that is diffused into the silver-selenide layer.
BRIEF SUMMARY OF THE INVENTION
[0008] Exemplary embodiments of the invention include a method of
forming a silver-rich silver-selenide layer by plating a silver
layer on a silver-selenide layer using an electroless process and
diffusing silver into the silver-selenide layer. Exemplary
embodiments of the invention also include a method of forming a
memory element. The memory element is formed by forming a first
electrode and forming a first layer of resistance variable material
over the first electrode. A silver-selenide layer is formed over
the first layer of resistance variable material and a silver layer
is plated on the silver-selenide layer by an electroless
process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The foregoing and other advantages and features of the
invention will become more apparent from the detailed description
of exemplary embodiments provided below with reference to the
accompanying drawings in which:
[0010] FIG. 1 illustrates a cross-sectional view of a memory
element fabricated in accordance with a first embodiment of the
invention and at an initial stage of processing;
[0011] FIGS. 2-7 illustrate a cross-sectional view of the memory
element of FIG. 1 at intermediate stages of processing;
[0012] FIG. 8 illustrates a cross-sectional view of a memory
element according to another exemplary embodiment of the invention;
and
[0013] FIG. 9 illustrates a processor-based system having a memory
element formed according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] In the following detailed description, reference is made to
various specific embodiments of the invention. These embodiments
are described with sufficient detail to enable those skilled in the
art to practice the invention. It is to be understood that other
embodiments may be employed, and that various structural, logical
and electrical changes may be made without departing from the
spirit or scope of the invention.
[0015] The term "substrate" used in the following description may
include any supporting structure including, but not limited to, a
semiconductor substrate that has an exposed substrate surface. A
semiconductor substrate should be understood to include
silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and
undoped semiconductors, epitaxial layers of silicon supported by a
base semiconductor foundation, and other semiconductor structures.
When reference is made to a semiconductor substrate or wafer in the
following description, previous process steps may have been
utilized to form regions or junctions in or over the base
semiconductor or foundation. The substrate need not be
semiconductor-based, but can be any support structure suitable for
supporting an integrated circuit. For example, the substrate can be
ceramic or polymer-based.
[0016] The term "silver" is intended to include not only elemental
silver, but silver with other trace metals or in various alloyed
combinations with other metals as known in the semiconductor
industry, as long as such silver alloy is conductive, and as long
as the physical and electrical properties of the silver remain
unchanged.
[0017] The term "silver-selenide" is intended to include various
species of silver-selenide, including some species which have a
slight excess or deficit of silver, for instance, Ag2Se, Ag2+xSe,
and Ag2-xSe.
[0018] The term "resistance variable memory element" is intended to
include any memory element, including Programmable Conductive
Random Access Memory (PCRAM) elements, which exhibit a resistance
change in response to an applied voltage.
[0019] The term "chalcogenide glass" is intended to include glasses
that comprise an element from group VIA (or group 16) of the
periodic table. Group VIA elements, also referred to as chalcogens,
include sulfur (S), selenium (Se), tellurium (Te), polonium (Po),
and oxygen (O).
[0020] Embodiments of the invention provide a method of forming a
chalcogenide material containing device, such as, and without
limitation, a resistance variable memory element, that does not
suffer from the drawbacks associated with conventional fabrication
methods. In accordance with the present invention, a silver layer
is deposited onto a silver selenide substrate using an electroless
plating bath.
[0021] Electroless plating of metal films onto different substrates
is a very important process in areas such as surface coating and
electronics fabrication. In general, electroless plating is the
deposition of a metal coating by immersion of a substrate in a
suitable bath containing a metal salt and a chemical reducing
agent. The metal ions are reduced by the reducing agent in the
plating solution and deposited on the substrate to a desired
thickness.
[0022] The electroless plating process, once initiated, is an
autocatalytic oxidation/reduction reaction, requiring only
occasional replenishment of the aqueous bath. The process resembles
electroplating in that the plating process may be run continuously
to build up a thick metal coating on the substrate. Electroless
plating differs from electroplating in that the electrons used for
reduction are supplied by a chemical reducing agent present in
solution. Thus, no outside current is needed for electroless
plating.
[0023] One attractive benefit of electroless plating over
electroplating is the ability to plate a substantially uniform
metallic coating onto a substrate having an irregular shape.
Frequently, electroplating an irregularly shaped substrate produces
a coating having non-uniform deposit thicknesses because of varying
distances between the cathode and anode of the electrolytic cell.
Electroless plating techniques do not exhibit this problem, as they
do not make use of electrolytic cells. Another advantage of
electroless plating is that electroless coatings are virtually
nonporous, which allows for greater corrosion resistance than
electroplated substrates.
[0024] The invention will now be explained with reference to the
figures, which illustrate exemplary embodiments and where like
reference numbers indicate like features. FIG. 1 depicts an initial
processing stage for the formation of a memory element according to
an exemplary embodiment of the invention. A portion of an optional
insulating layer 12 is formed over a semiconductor substrate 10,
for example, a silicon substrate having circuitry fabricated
thereon. It should be understood that the memory elements of the
invention can be formed over a variety of substrate materials and
not just semiconductor substrates such as silicon, as shown above.
For example, the optional insulating layer 12 may be formed on a
ceramic or polymer-based substrate. The insulating layer 12 may be
formed by any known deposition methods, such as sputtering by
chemical vapor deposition (CVD), plasma enhanced CVD (PECVD) or
physical vapor deposition (PVD). The insulating layer 12 may be
formed of a conventional insulating oxide, such as silicon oxide
(SiO.sub.2), a silicon nitride (Si.sub.3N.sub.4), or a low
dielectric constant material, among many others.
[0025] A first electrode layer 14 is formed over the insulating
layer 12, as also illustrated in FIG. 1. The first electrode layer
14 may comprise any conductive material, for example, tungsten,
nickel, tantalum, aluminum, or platinum, among many others. A first
dielectric layer 15 is formed over the first electrode 14. The
first dielectric layer 15 may comprise the same or different
materials as those described above with reference to the insulating
layer 12.
[0026] Referring now to FIG. 2, an opening 13 extending to the
first electrode layer 14 is formed in the first dielectric layer
15. The opening 13 may be formed by any method, such as, by
conventional photolithographic processes.
[0027] A chalcogenide glass layer 17 is formed over the second
insulating layer 15, to fill in the opening 13, as shown in FIG. 3.
According to an embodiment of the invention, the chalcogenide glass
layer 17 can be germanium-selenide glass having a
Ge.sub.xSe.sub.100-x stoichiometry. The preferred stoichiometric
range is between about Ge.sub.20Se.sub.80 to about
Ge.sub.43Se.sub.57 and is more preferably about Ge.sub.40Se.sub.60.
The chalcogenide glass layer 17 preferably has a thickness from
about 100 Angstroms (.ANG.) to about 1000 .ANG. and is more
preferably about 150 .ANG..
[0028] The formation of the chalcogenide glass layer 17, having a
stoichiometric composition in accordance with the invention, may be
accomplished by any suitable method. For instance,
germanium-selenide glass can be formed by evaporation,
co-sputtering germanium and selenium in the appropriate ratios,
sputtering using a germanium-selenide target having the desired
stoichiometry, or chemical vapor deposition with stoichiometric
amounts of germanium tetrahydride (GeH.sub.4) and selenium hydride
(SeH.sub.2) gases (or various compositions of these gases), which
result in a germanium-selenide film of the desired stoichiometry,
are examples of methods which may be used.
[0029] As shown in FIG. 4, a silver-selenide layer 18 is deposited
on the surface of the chalcogenide glass layer 17. A variety of
processes can be used to form the silver-selenide layer 18. For
instance, physical vapor deposition techniques such as evaporative
deposition and sputtering may be used. Other processes such as
chemical vapor deposition, co-evaporation or depositing a layer of
selenium above a silver layer to form the silver-selenide layer 18
can also be used.
[0030] The use of a metal containing layer, such as a
silver-selenide layer 18, in contact with the chalcogenide glass
layer 17 makes it unnecessary to photodope the glass with silver.
As an optional variant, however, it is possible to also metal dope
using, for example, silver, the chalcogenide glass layer 17, which
is in contact with the silver-selenide layer 18.
[0031] Preferably, the thickness of layers 17, 18 is such that a
ratio of the silver-selenide layer 18 to the chalcogenide glass
layer 17 thicknesses is between about 5:1 and about 1:1. In other
words, the thickness of the silver-selenide layer 18 is between
about 1 to about 5 times greater than the thickness of the
chalcogenide glass layer 17. Even more preferably, the ratio is
between about 3.1:1 and about 2:1.
[0032] Referring now to FIG. 5, a silver layer 50 is plated on the
silver-selenide layer 18 by an electroless process. The electroless
plating process is less energetic than the conventionally used
processes (e.g., sputtering and evaporation), which would cause an
unknown amount of silver to be incorporated into the
silver-selenide layer 18 during the energetic deposition process.
Accordingly, by plating the silver layer 50 on the silver-selenide
layer 18 using an electroless process, the amount of silver to be
incorporated in the silver-selenide layer 18 can be better
controlled.
[0033] In the illustrated embodiment, the silver layer 50 is plated
to a thickness within the range of approximately 50 .ANG. to
approximately 250 .ANG., and preferably to approximately 200 .ANG..
To plate the silver layer 50 onto the silver-selenide layer 18, the
silver-selenide layer 18 is first activated using an appropriate
activation chemistry. The activation chemistry allows the plating
of the silver onto the silver-selenide layer 18.
[0034] According to one exemplary embodiment, the silver-selenide
layer 18 can be activated by exposure to a nickel (Ni) and gold
(Au) colloidal mixture (e.g., Ronamerse) for approximately 5
minutes. According to another exemplary embodiment, the
silver-selenide layer 18 can be activated by exposure to colloidal
palladium (Pd) particles for approximately 5 minutes.
Alternatively, the silver-selenide layer 18 can be activated by
exposure to a solution of 500 ml water; 0.5 g palladium chloride
(PdCl.sub.2), and 1 ml hydrogen fluoride (HF) for approximately 90
seconds. According to yet another exemplary embodiment, the
silver-selenide layer 18 can be activated by first exposing the
layer 18 to a solution of dimethylethylenediamine for approximately
2 minutes and then, exposing the layer 18 to a solution of 500 ml
water, 0.5 g palladium chloride, and 1 ml hydrogen fluoride for
approximately one minute.
[0035] If desired, prior to activation, the silver-selenide layer
18 can be pretreated to prepare the silver-selenide layer 18 for
activation. Such a pretreatment can include a cleaning step. For
example, the silver-selenide layer 18 can be exposed to a mixture
of ammonium fluoride (NH.sub.4F) and phosphoric acid
(H.sub.3PO.sub.4). An exemplary mixture is available under the name
Blend B from General Chemical.
[0036] Once the silver-selenide layer 18 is properly activated, it
is exposed to a plating solution for a period of time. The amount
of time the silver-selenide layer 18 is exposed to the plating
solution can be adjusted to achieve the desired thickness of the
plated silver layer 50. Preferably, the activated silver-selenide
layer 18 is exposed to the plating solution for a period of time
sufficient to deposit a silver layer 50 having a thickness within
the range of approximately 50 .ANG. to approximately 250 .ANG., and
more preferably approximately 200 .ANG.. According to exemplary
embodiments of the invention, the electroless plating solution
includes water, a water soluble compound containing silver (e.g.,
silver nitrate (AgNO.sub.3), among others), a chelating agent
(e.g., tartrate, Ethylenediaminetetraacetic Acid (EDTA), among
others) that prevents chemical reduction of the metal ions in
solution while permitting selective chemical reduction on a surface
of the substrate, and a chemical reducing agent (e.g., imidazole,
glucose, hydrazine, among others). Additionally, the plating
solution may include a buffer for controlling pH and various
optional additives, such as bath stabilizers and surfactants.
[0037] A first exemplary plating solution includes: 100 milliliters
(ml) water (H.sub.2O); 5 ml ammonium hydroxide (NH.sub.4OH); 3
grams (g) ammonium sulfate ((NH.sub.4)SO.sub.4); 1.25 g silver
nitrate (AgNO.sub.3); and 2 g tartrate. Optionally, an additional
reducing agent, such as ammonium hypophosphite
((NH.sub.4).sub.3PO.sub.2), can also be added. A second exemplary
plating solution includes: 150 ml water; 0.81 g silver nitrate; 5.5
g ammonium hydroxide (NH.sub.4OH); 4.3 5ml acetic acid
(CH.sub.3COOH); and approximately 0.2 to approximately 2 g EDTA.
Optionally, an additional reducing agent, such as hydrazine can be
added. Preferably approximately 70 microliters (.mu.l) of hydrazine
are added to the second exemplary plating solution. According to
another embodiment of the invention, a third exemplary plating
solution can include: 150 ml water; 0.8 g silver nitrate; 2.4 g
succinimide; 1.8 g imidazole; and 1 ml ammonium hydroxide.
Optionally, an additional reducing agent, such as hydrazine can be
added.
[0038] When the plating solution according to the specific
embodiment described above is used and layer 18 is activated
according to a specific embodiment described above, the plating
time is preferably within the range of approximately 10 minutes to
approximately 15 minutes. Additionally, the plating solution is
preferably at a temperature within the range of approximately
55.degree. C. to approximately 60.degree. C.
[0039] Once all of the components are combined in a suitable
container, the water soluble silver salt dissolves, releasing
silver ions into the solution. The complexing agent strongly binds
with the silver ions, preventing them from being reduced in the
solution, but permitting reduction of the silver on the activated
silver-selenide layer 18 surface. Specifically, the activated
silver-selenide layer 18 surface acts as a catalyst, allowing the
reduction of silver ions to metallic silver, which is deposited on
the surface of the activated silver-selenide layer 18.
[0040] Subsequently, silver from the silver layer 50 is diffuses
into the silver-selenide layer 18. Diffusion of the silver from
silver layer 50 causes an excess of silver in the silver-selenide
layer 18, making the silver-selenide layer 18 silver-rich
(Ag.sub.2+xSe).
[0041] As shown in FIG. 6, an optional conductive adhesion layer 30
is formed over the silver layer 50 and a top electrode 22 is formed
over the conductive adhesion layer 30. Suitable materials for the
conductive adhesion layer include conductive materials capable of
providing good adhesion between the silver layer 50 and the top
electrode layer 22. Desirable materials for the conductive adhesion
layer 30 include chalcogenide glasses.
[0042] The top and bottom electrodes 22, 14 can be any conductive
material, such as tungsten, tantalum, aluminum, platinum, silver,
and conductive nitrides. The bottom electrode 14 is preferably
tungsten. The top electrode 22 is preferably tungsten or tantalum
nitride.
[0043] The conductive adhesion layer 30 may be the same
chalcogenide glass material used in the chalcogenide glass layer 17
discussed above. In this case, the conductive adhesion layer 30 can
be formed by sputtering the chalcogenide glass onto the silver
layer 50. A small amount of silver from the silver layer 50 is
incorporated into the chalcogenide glass adhesion layer 30 when
sputter deposited over the silver layer 50 due to the energetic
nature of the sputtering process. Thus, the top electrode 22 shorts
to the chalcogenide glass adhesion layer 30, creating a conductive
path from the top electrode 22 to the first glass layer 17. The
desired thickness of a chalcogenide glass conductive adhesion layer
30 is about 100 .ANG..
[0044] Use of a conductive adhesion layer 30 between the silver
layer 50 and the top electrode 22 can prevent peeling of the top
electrode 22 material during subsequent processing steps such as
photoresist stripping. Electrode 22 materials, including tungsten,
tantalum, tantalum-nitride, and titanium, among others, may not
adhere well to silver layer 50. For example, adhesion between the
two layers 50, 22 may be insufficient to prevent the electrode 22
layer from at least partially separating (peeling) away from the
underlying silver layer 50 and thus losing electrical contact with
the underlying layers 18, 17, 14 of the memory element 100. Poor
contact between the top electrode 22 and the underlying memory
element layers 18, 17, 14 can lead to electrical performance
problems and unreliable switching characteristics. Use of a
conductive adhesion layer 30 can substantially eliminate this
problem.
[0045] Use of the silver-selenide layer 18 in contact with the
chalcogenide glass layer 17 can eliminate the need to dope the
chalcogenide glass layer 17 with a metal during formation of the
memory element 100. The silver-selenide layer 18 provides a source
of silver-selenide, which is driven into chalcogenide glass layer
17 by a conditioning step after formation of the memory element 100
(FIG. 11). Specifically, the conditioning step comprises applying a
potential across the memory element structure 100 such that
silver-selenide from the silver-selenide layer 18 is driven into
the chalcogenide glass layer 17, forming a conducting channel.
Movement of Ag.sup.+ ions into or out of that channel causes an
overall resistance change for the memory element 100. The
conditioning potential generally has a longer pulse width and
higher amplitude than a potential used to program the memory
element. After the conditioning step, the memory element 100 may be
programmed.
[0046] Referring now to FIG. 7, an optional tungsten nitride layer
26 may be formed over the second electrode material 22. One or more
additional dielectric layers 16 may be formed over the second
electrode 22 or alternatively over the tungsten nitride layer 26
and the first dielectric layer 15 (as shown) to isolate the
resistance variable memory element 100 from other structures
fabricated over the substrate. Conventional processing steps can
then be carried out to electrically couple the second electrode 22
to various circuits of memory arrays.
[0047] The embodiments described above refer to the formation of
only a few possible resistance variable memory element 100
structures in accordance with the invention. It must be understood,
however, that the invention contemplates the formation of other
such resistance variable memory elements, which can be fabricated
as a memory array and operated with memory element access
circuits.
[0048] The memory element 100 show in FIG. 7 is exemplary only.
Accordingly, a memory element 100 formed according to the invention
can include additional layers. For example, as shown in FIG. 8, the
memory element 100 can include a second silver layer 40 below the
silver selenide layer 18. Preferably, the silver layer 40 has a
thickness of approximately 50 .ANG.. The silver layer 40 can be
formed by any suitable method, such as sputtering and
evaporation.
[0049] FIG. 9 illustrates a typical processor system 900 which
includes a memory circuit 948, for example a programmable conductor
RAM, which employs resistance variable memory elements fabricated
in accordance with the invention. A processor system, such as a
computer system, generally comprises a central processing unit
(CPU) 944, such as a microprocessor, a digital signal processor, or
other programmable digital logic devices, which communicates with
an input/output (I/O) device 946 over a bus 952. The memory 948
communicates with the system over bus 952 typically through a
memory controller.
[0050] In the case of a computer system, the processor system 900
may include peripheral devices such as a floppy disk drive 954 and
a compact disc (CD) ROM drive 956, which also communicate with CPU
944 over the bus 952. Memory 948 is preferably constructed as an
integrated circuit, which includes one or more resistance variable
memory elements 100 (FIGS. 1-8). If desired, the memory 948 may be
combined with the processor, for example CPU 944, in a single
integrated circuit.
[0051] The above description and drawings are only to be considered
illustrative of exemplary embodiments which achieve the features
and advantages of the invention. Modification and substitutions to
specific process conditions and structures can be made without
departing from the spirit and scope of the invention. Accordingly,
the invention is not to be considered as being limited by the
foregoing description and drawings, but is only limited by the
scope of the appended claims.
* * * * *